The grease pencil merge depth shader is designed to only work correctly
in octographic mode. The uv coordinates used `noperspective` attribute.
Somehow this doesn't lead to render artifacts on most platforms and was
only detected on OSX + AMD cards.
This fix would calculate the uv coordinate inside the fragment shader
and isn't passed along from the vertex shader.
Thanks to Sebastián Barschkis for providing the hardware and time and
Clément Foucault for helping out with the final fix.
cacc1d723c